A so-called position sensor-less control device that controls the driving of an electric motor without the use of a position sensor for detecting a magnetic pole position of a rotor has been known. Up to now, there is a high frequency voltage estimation method that applies a high frequency voltage higher than a frequency of a drive voltage of the electric motor to the electric motor, to thereby estimate the magnetic pole position of the rotor. First, the high frequency voltage estimation method will be described.
In a salient pole type synchronous motor, an inductance of an armature coil is minimized in a d-axis direction on a rotational orthogonal coordinate (dq axis coordinate) system. Therefore, when a high frequency voltage rotating with a predetermined amplitude at a predetermined frequency is applied to the armature coil, a trajectory of the high frequency current on a fixed orthogonal coordinate (αβ axis coordinate) system at that time is presented as shown in FIG. 11. FIG. 11 is a graph showing a trajectory of a high frequency voltage Vh and a trajectory of a high frequency current Ih in the αβ axis coordinate system. As shown in FIG. 11, the trajectory of the high frequency current Ih has an elliptical shape. Hereinafter, the ellipse is also referred to as a current elliptical trajectory. In FIG. 11, the magnetic pole position of the rotor is determined as an angle θ formed by a d-axis with respect to an α-axis. In this case, when it is assumed that an α-axis component and a β-axis component of a maximum value Imax of the amplitude of the high frequency current Ih in the current elliptical trajectory of one cycle of the high frequency voltage are “Imaxα” and “Imaxβ”, respectively, a relationship of “tan θ=Imax β/Imax α” is established. With the use of the above relationship, the magnetic pole position θ of the rotor can be calculated based on the detection values of “Imax α” and “Imax β”.
In the case where the current elliptic trajectory has a symmetrical shape as shown in FIG. 11, in one cycle of the high frequency voltage Vh, there are two maximum amplitude values Imax1 and Imax2 in the major axis direction of one cycle of the current elliptical trajectory. The “Imax1” is a maximum value on a positive side in the major axis direction, in other words, a maximum amplitude value in an N pole direction of the major axis direction. The “Imax2” is a maximum value on a negative side in the major axis direction, in other words, a maximum amplitude value in an S pole direction of the long axis direction. In other words, two maximum values of the high frequency current Ih are extracted from one cycle of the current elliptical trajectory.
Conveniently, in a PM synchronous motor (magnet rotor synchronous motor), in a situation where a magnetic circuit has a magnetic saturation tendency responsive to the high frequency current, when a direction of a magnetic flux caused by the high frequency current matches a direction of the magnet flux of the rotor, an inductance of the armature coil decreases as compared with the case where those directions are opposite to each other. A long diameter of the current elliptical trajectory is increased as the inductance of the armature coil is decreased. In other words, as shown in FIG. 12, the current elliptical trajectory has an irregular shape extending in the N pole direction of the major axis direction. For that reason, if the maximum amplitude value is selected from each data of the high frequency current Ih for one cycle of the current elliptical trajectory, the maximum amplitude value Imax2 in the S pole direction of the major axis direction is naturally excluded and the maximum amplitude value Imax1 in the N pole direction of the major axis direction can be extracted. Therefore, the magnetic pole position θ of the rotor can be estimated through the above arithmetic expression based on the extracted maximum amplitude value Imax 1 in the N pole direction of the major axis direction.
However, when the estimation method of the above type is used, if a difference between the maximum amplitude value Imax1 in the N pole direction of the major axis direction and the maximum amplitude value Imax2 in the S pole direction of the major axis direction is small, there is a possibility that the “Imax2” is erroneously determined as the maximum amplitude value in the N pole direction of the major axis direction. In that case, if the magnetic pole position θ of the rotor is calculated on the basis of the “Imax2”, there is a possibility that the magnetic pole position of the rotor is erroneously determined to be deviated by 180° in an electric angle.
Therefore, in the control device disclosed in Patent Literature 1, a difference value ΔI (=Imax1−Imax2) between one maximum amplitude value Imax1 and the other maximum amplitude value Imax2 is calculated, and the amplitude of the high frequency voltage is adjusted so that the difference value ΔI falls within a preferable range. As a result, a state in which the difference between one maximum amplitude value Imax1 and the other maximum amplitude value Imax2 is large can be maintained. Therefore, when the maximum amplitude value is selected from each data of the high frequency current Ih for one cycle of the current elliptical trajectory, the maximum amplitude value Imax1 in the N pole direction of the major axis direction can be more reliably extracted. Therefore, an estimation accuracy of the magnetic pole position θ of the rotor can be improved.
Incidentally, in order to increase the estimation accuracy of the magnetic pole position of the rotor in the control device disclosed in Patent Literature 1, the amplitude of the high frequency voltage needs to be increased over an entire range of the electric angle. As a result, the amplitude of the high frequency current also increases. When the amplitude of the high frequency current increases, for example, there is a possibility that the amount of heat generated by a drive circuit (inverter circuit) of the electric motor and the amount of heat generated by the electric motor per se may increase.